1. Field of the Invention
The present invention relates to an optical pickup capable of performing data recording and writing by irradiating an optical disc with a light beam, and more specifically, to a technology for correcting wave aberration (e.g., spherical aberration and astigmatism) so as to improve the quality of data reading and writing.
2. Description of Related Art
Optical discs such as compact discs (hereinafter, CDs) and digital versatile discs (hereinafter, DVDs) have been widely used. Moreover, in recent years, research has been carried out on increasing the recording density of optical discs aiming at increasing the amount of data that can be recorded on optical discs, and optical discs such as HD-DVDs and blue-ray discs (hereinafter, BDs) on which can be recorded a large amount of data have started to be put to practical use.
In performing recording and reproduction with respect to such optical discs, an optical pickup capable of performing data reading and writing by irradiating an optical disc with a light beam is used. Here, the wavelength of the light beam and the numerical aperture (NA) of an objective lens (which has a function of condensing a light beam emitted from a light source onto a recording layer of an optical disc) need to be changed according to the kind of the optical disc.
For example, the wavelengths used for CDs, DVDs, and BDs are 780 nm, 650 nm, and 405 nm, respectively. The NAs of the objective lenses used for CDs, DVDs, and BDs are 0.45, 0.65, and 0.85, respectively.
As described above, in an optical pickup, light beams having different wavelengths and objective lenses having different NAs need to be used for different kinds of optical discs, and one possible way of achieving this is to use different optical pickups for different kinds of optical discs. However, it is more convenient if a single optical pickup is capable of performing, for example, data reading with respect to a plurality of kinds of optical discs, and thus there have conventionally been developed many optical pickups that are compatible with a plurality of kinds of optical discs.
Some optical pickups compatible with a plurality of kinds of optical discs have only one objective lens for condensing a light beam emitted from a light source onto a recording layer of an optical disc, and others have a plurality of objective lenses. Some of the optical pickups that are compatible with a plurality of kinds of optical discs and that have a plurality of (for example, two) objective lenses are structured such that there are formed independent optical paths through which light beams emitted from light sources travel to reach the objective lenses. With this structure, the size of an optical pickup and the number of optical members needed in an optical pickup increase, and this leads to a higher cost.
In this regard, Patent Document 1 proposes an optical pickup that has two objective lenses and a raising prism disposed in an optical system, the raising prism having a first surface that reflects laser light having a wavelength of 660 nm or 780 nm while it transmits laser light having a wavelength of 405 nm, and a second surface that reflects laser light having a wavelength of 405 nm. With this structure, the optical system of an optical pickup can be formed such that optical paths each formed corresponding to one of the objective lenses have a portion thereof in common, instead of being completely independent of each other. Thus, this structure is advantageous in coping with the above mentioned problems, that is, increase in size and cost of an optical pickup.
Optical pickups compatible with a plurality of kinds of optical discs and optical pickups that can handle a multilayer optical disc having a plurality of recording layers disadvantageously suffer from spherical aberration (which is a kind of wave aberration) that occurs due to the difference in thickness between protective layers (including a transparent layer laid between the recording layers, the same shall apply hereinafter) for protecting the recording layers. Thus, various spherical aberration correcting mechanisms for correcting the just described spherical aberration have conventionally been proposed.
Examples of such spherical aberration correcting mechanisms include one that corrects spherical aberration with, for example, a liquid crystal device by changing the phase distribution of a light beam passing through the liquid crystal device, and one that corrects spherical aberration by adjusting the divergence/convergence angle of a light beam that enters an objective lens (see, for example, Patent Document 2).
Patent Document 1: JP-A-2005-196859
Patent Document 2: JP-A-2007-52864